This invention relates to a system for preventing shoreline erosion and/or causing sand accretion on the beach.
In many shore areas of the world the shoreline is rapidly receding because of the rise of sea level and the movement of the currents and waves especially as the result of storms. This movement of currents and waves gives rise to a corresponding movement of particles, such as sand particles, in the shore region. This movement of particles proceeds by three main processes.
The first process is known as "bed load transportation" and results in transportation of a large amount of sand deposits by the rolling or slipping of said particles in the boundary layer near the bottom.
A second process is known as "transportation by suspension". In this process, the lightest particles are moved from the bottom by turbulence of the water and become suspended in the fluid. The particles are carried a long distance from their origins before being deposited.
The third process, which also produces a localized transportation of the particles, is known as "saltation". In saltation, particles jump up from the bottom and fall back again close to its original location.
Shoreward movement of particles, or sediment accretion, is predominantly by "bed load transportation" whereas a seaward motion, erosion, is caused predominantly by suspension. Extensive turbulence, which results in a suspension of the particles, occurs in a breaking wave and in the surf zone.
Various means have been designed to combat erosion of beaches, shorelines, with varying degrees of success. These means have taken two main forms, wave barriers (relatively solid structures) and artificial seaweed.
In U.S. Pat. No. 1,516,767, a baffle, having various shapes, was implanted on or near the ocean floor and its purpose was to deflect the minor ocean currents and break up the major currents. U.S. Pat. No. 3,276,210, provides a series of circular or triangular shaped barriers, suspended mainly at the surface or near the surface of the wave, with the main purpose of breaking up an incoming wave. The main purpose of this barrier is to break up the waves and protect moorings.
Other forms of barriers which are utilized to break up waves are shown in U.S. Pat. Nos. 3,628,334; 3,673,805; 3,691,774; and 3,848,419.
Katayma, U.S. Pat. No. 3,386,250, utilizes a series of concrete blocks in the shape of isosceles triangles to apparently control the direction of the current flow and reduce the eddy currents and turbulence which should result in reduced erosion. Fair, U.S. Pat. No. 3,894,397, adds a flapper-type valve to concrete blocks which closes against seaward wave motion. This structure thus favors a shoreward wave motion over a seaward wave motion that would favor an accretion of the shoreline.
Tibbett, U.S. Pat. No. 3,892,075, is a "vortex generating system" which causes a depositing of suspended particles.
Larsen, U.S. Pat. No. 3,928,978, suspends a mat over a portion of the sea floor at a specific distance from the sea floor. Sediment flow is impeded by this device resulting in an accumulation under the mat. When the sediment level reaches that of the mat, the mat is raised and the process repeated.
Still other forms of wave barriers may be found in U.S. Pat. Nos. 3,846,990; 3,267,679; and 4,130,994.
Many of the described wave barriers are massive structures which cannot be easily moved. They also tend to be rigid. They are designed primarily to break up the wave energy and eventually succeeds to prevent some erosion.
The second main means for controlling erosion is by the utilization of devices generally known as artificial seaweeds.
Various forms of artificial seaweeds were developed to avoid the disadvantages exhibited by the barrier type structures, and are illustrated by U.S. Pat. Nos. 3,299,640; 3,323,310; 3,590,585; 3,559,407 and 4,221,500.
DeWinter, U.S. Pat. No. 3,590,585, interweaves strands of a thermoplastic material, to prevent the knotting and entangling of individual strands, into a net form. The net is anchored by a tube filled with cement or sand. The density of the net fibers are such that the net will float in the water.
Nielsen, U.S. Pat. No. 3,299,640, is directed to a wide screen formed by a large series of filamentary plastic elements, strands, which are secured at one end to an anchoring means. Each row of the filamentary strands may be attached to a single anchoring means and several rows placed on the sea floor or several rows of filamentary strands may be attached to the same anchoring means.
Arpin, U.S. Pat. No. 3,323,310, is directed to a simulated reef which is composed of an elongated structure, which may be concrete, in which is imbedded a large number of discrete extending rods, tubes or wands of flexible material.
Garrett, U.S. Pat. No. 4,221,500, is directed to a flexible, non-woven sheet material which is attached to an anchor and which has series of parallel cuts extending from its upper edge down to approximately the anchoring means. An uncut sheet of material separates the cut area from the anchoring means.
These long, flexible, usually symmetrical, means, although they alleviate some of the disadvantages of the bulky barrier means are still inefficient in preventing erosion and/or accretion, and are subjected to large wave forces during storms.